In modern wireless communication systems, especially in a Code Division Multiple Access (CDMA) wireless communication systems, smart antennas have been one of the most attractive technologies. By means of smart antenna arrays and the technology based on digital signal processing, wireless base stations can achieve self-adaptive beam forming of both transmitting and receiving signals, therefore greatly reducing system interference, increasing system capacity, decreasing the transmitting power and improving the receiving sensitivity.
In the Chinese patent for invention titled, “A Time-Division Duplex Synchronous Code Division Multiple Access Wireless Communication System with Smart Antenna” (CN 97 104039.7), the structure of a wireless communication system base station with a smart antenna is disclosed. The base station comprises an antenna array with one or more than one antenna elements, radio frequency (RF) cables and RF transceivers correspondingly connected. Based on the signals received from the user terminal in each antenna element of the antenna array, a baseband signal processor can obtain the space vector characteristics and direction of arrival (DOA) of the uplink signals, from which the weights of every link obtained are employed for downlink transmitting beam forming. In this way, all the functionality of a smart antenna is achieved under the circumstances of symmetrical radio wave propagation characterized as the result of time-division duplex communication.
In order to transmit and receive signals accurately with the smart antenna, every antenna element, RF cable and transceiver, which comprise the smart antenna must operate without difference, (i.e. every transmitting and receiving link should have the same amplitude and phase response). The procedure and method for amplitude and phase compensation of each transmitting and receiving link comprise the smart antenna calibration relating to the present invention.
As the characteristics of electronic elements, especially active elements, differ from each other, the sensitivities thereof to operation frequency and ambient temperature are different, and the changes in the characteristics of every transmitting and receiving link due to the reasons above are different, the smart antenna calibration of the present invention should generally be carried out periodically while the base station is in operation.
In the published document of the Chinese patent titled, “A Method And Apparatus For Calibrating A Smart Antenna Array” (CN 99 111350.0) (See FIG. 1), a calibrating link is set by an antenna element 201, a couple structure 205, a RF cable 206 and a pilot transceiver 207 sequentially connected. The couple structure 205 sets up a RF couple connection with all the antenna elements 201-1, 201-2 . . . 201-N of the smart antenna, and allocates the RF signals to all antenna elements comprising the array according to need. The pilot transceiver 207 has the same structure as the other transceivers 203-1, 203-2 . . . 203-N of the base station and uses a common local oscillator 208. The pilot transceiver 207 works coherently with other transceivers and connects with the baseband signal processor 204 via a digital bus. Each antenna element connects to a RF cable and further to a transceiver, and the connected antenna element, RF cable and transceiver form a transmitting link or a receiving link. Ac, A1, A2, . . . AN in FIG. 1 represent the connection points between the antenna elements and the RF cables 201-1, 201-2 . . . 201-N, respectively; BC, B1, B2 . . . BN represent the connection points of the pilot transceiver 207 and the radio transceivers 203-1, 203-2 . . . 203-N with the baseband signal processor 204, respectively.
When making calibrations, the calibration link is first calibrated by using a network vector analyzer and recording the receiving and transmitting transmission coefficients of the calibration link respectively, then performing the receiving calibration and transmitting calibration respectively. In the receiving calibration, the pilot transceiver transmits a signal at a given working frequency, and all the other links in the base station are set in the receiving state. The outputs of all the receiving links are measured and the ratio of the receiving transmission-coefficient (vector) of each link to the transmission-coefficient (vector) of a reference link is computed. When the ratio of the amplitudes of the transmission-coefficients equals to 1, the phase difference of each receiving link from the reference link is recorded. In the transmitting calibration, set one link after another of the base station in the transmitting state with all the other links closed at the same time as the pilot transceiver receives the signal of each transmitting link at a given working frequency, respectively; the ratio of the transmission-coefficient (vector) of each link during transmission to the transmission-coefficient (vector) of the reference link is computed, and when the ratio of the amplitudes of the transmission-coefficients equals 1, the phase difference of every receiving link from the reference link is recorded.
The patent mentioned above only relates to the general scheme of the method and apparatus for real-time calibration without a specific engineering implementation thereof, including the calibration sequence used in the transmitting and receiving calibration and the computation by the baseband signal processor, and how to perform the real-time calibration when the smart antenna is in operation. In addition, the transmitting calibration as described above is carried out with one link in the transmitting state at a time while all other links are in the receiving state, which is unfavorable for fast real-time calibration.